Such particle filters are needed to clean exhaust gases of internal combustion engines, especially in vehicles operated with diesel fuel, in order to filter out the soot particles which are hazardous for health and represent major and dangerous environmental pollution. These particle filters have been known in a plurality of designs. In one design, the exhaust gas is filtered while passing through a ceramic block (monolith) that has a plurality of passage canals, of which one canal is closed on the inlet side and another is closed on the outlet side in a checkerboard pattern, so that the exhaust gas will flow into one canal, after which it will flow into the adjacent canal through the surrounding monolith section acting as the filter, and will leave the latter canal freed from soot particles. Such a soot filter is disclosed in German Offenlegungsschrift No. DE-0S 32,17,357.
Filter cartridges are used to filter out particles in another embodiment. These filter cartridges are preferably designed as wound filters. A plurality of layers of filter material are wound around a support tube provided with exhaust gas outlet openings, so that an element comparable to a textile thread spool is obtained. Another arrangement is shown in German Offenlegungsschrift No. DE-OS 38,15,148, with special representation of the mounting of the filter cartridges in mounting plates. The filter cartridges can also be formed by a tube of filter material pulled over a support tube corresponding to West German Offenlegungsschrift No. DE-OS 38,23,205. The filter cartridges are passed through from the outside to the inside. The exhaust gas to be cleaned enters the support tubes, which are closed on the inlet side, through the filter material. The soot particles are retained during flow through the wound filter material, and the cleaned exhaust gas flows through the support tubes and into the outlet funnel, and is discharged there. The filter cartridges are arranged on concentric circles in the filter housing and have a uniform design. Instead of the inlet-side mounting plate provided with exhaust gas passage openings, it is also possible to provide a mounting grid.
During the operation of the internal combustion engine, soot buildup takes place during the flow through the particle filter, i.e., the filter material increasingly becomes clogged with soot particles, and the soot must be removed after a relatively short operating time. Mechanical removal is practically ruled out, because it would require disassembly of the entire filter and removal of the soot. Therefore, it is necessary to resort to burning free the accumulated soot. To do so, oxidizing agents are added as additives to the exhaust gas from a supply container, so that the soot collected comes into contact with these agents, which lower the ignition point of the soot and increase the rate of combustion, and it can be burned off at relatively low exhaust gas temperatures. However, this type of burning free of the soot collected has the disadvantage that the additives are readily inflammable chemical compounds and introduce into the exhaust gas undesirable components, whose environmental safety has not yet been proven. Attempts have therefore been made to connect a diesel burner as an external heat source, but it was unable to regenerate the filters with the prior-art arrangement during the driving operation of the vehicle. Therefore, very large filters are needed with this solution in order to reach a storage capacity that permits longer travel between the regeneration cycles. This resting-time regeneration is possible in vehicles operated intermittently, e.g., buses in municipal transportation systems. In other vehicles, the regeneration must be performed during driving operation. Therefore, e.g., two filters are arranged in parallel to one another, and one of the two filters is being regenerated at any given time.
In devices with only one particle filter having a plurality of filter cartridges, it is also known that for "regeneration during driving operation", an external heat source is connected as soon as a heavier coating with soot particles can be determined, e.g., from the exhaust gas back pressure, and this external heat source heats the exhaust gas to be cleaned sufficiently (&gt;600.degree. C.) so that the burning-free process can take place. However, it was found that the filter cartridges are not burned free over their entire length, especially in the case of loads involving a weak exhaust gas mass flow. This is due to the fact that even burning free of approximately the front half of the filter cartridges ensures a sufficiently low flow resistance, so that there is no sufficient flow through the rear part or the outermost filter cartridges. This incomplete regeneration leads to progressively shorter soot buildup times, which can lead to failure of the filter in the worst case. Experiments have shown that the soot buildup time decreased from, e.g., 135 minutes after the first soot build-up to 15-20 minutes.
In order to bring about a substantial improvement in regeneration burning free, an improvement of burning free was achieved according to German patent application No. P 40,04,861.6, which was not previously published, by arranging in the space between the filter cartridges so-called overflow tubes, via which the exhaust gas to be cleaned is fed to the filter cartridges such that uniform admission over the entire length of the filter cartridges is achieved.
A device of this type has become known from DE 38,36,697 Al, in which the mounting plates together with the filter cartridges form a rotatable drum, so that one segment of this drum can be brought at any time into a zone in which the burning-free takes place. Such a device is very complicated, especially in terms of the mounting of the rotatable drum and the sealing to prevent bypass flow of uncleaned exhaust gas. These problems will be solved in this document. However, such devices with segment-by-segment free-burning are not suitable for mass production installation.